Heat oven to 350°. Coat a 13 x 9 x 2-inch baking pan with nonstick cooking or baking spray.
Cake. Prepare cake mix with 1 1/4 cups water, the eggs and oil as per package directions. Transfer batter to prepared pan. Bake at 350° for 31 to 33 minutes, or as per package directions. Cool completely.
Trim crowned area from top of cake; reserve scraps for later use. Invert cake onto serving platter.
Frosting and decorations. Place chopped chocolate in a medium bowl. Heat cream in a glass measuring cup in microwave for 1 minute. Pour over chocolate and let stand 5 minutes. Whisk until smooth. Let cool an additional 5 minutes.
Transfer 1/2 cup of the chocolate mixture to a pastry bag or resealable plastic bag. Pour and spread remaining frosting over cake, allowing some to drip down cake sides. Snip a small corner from plastic bag. Pipe decorations onto cookies. Let dry. With a knife, cut slits into cake and insert decorated cookies into slits. Crumble reserved cake scraps and place on cake in front of cookies. Scatter pumpkin candies over cake.